Reduced pellets

ABSTRACT

A carbonaceous material is controlled such that the amount of carbon is from 7 to 60 mass % based on the total amount of iron and Zn in a starting mixture comprising one or more of ducts containing iron oxide and Zn oxide and a binder in an amount to bond the dusts, and water is added to prepare green pellets incorporated with the carbonaceous material. Then, dry pellets prepared by drying the thus prepared green pellets into a reduction furnace, the dry pellets are heated by heat transfer, mainly, radiation such that a temperature elevation rate is from 3 to 13° C./sec within a temperature range from 150 to 900° C. of the pellets, thereby reducing Zn oxide and evaporating Zn, as well as reducing iron oxide to produce reduced iron pellets.

This application is a Continuation of application Ser. No. 09/213,249Filed on Dec. 17, 1998 now U.S. Pat. No. 6,152,983.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a method of producing reduced ironpellets formed by reducing iron oxide pellets containing Zn ingredients.More in particular, the invention relates to a method of producingreduced iron pellets formed by reducing iron oxide pellets containingdusts (including sludges) containing Zn ingredients formed in ironworks.

2. Description of the Related Art

In iron and steel making processes including blast furnaces, convertersand electric furnaces, various kinds of iron oxide-containing dusts orsludges are formed in a great amount which are recovered and reused asiron sources (used in the form of pellets or sintered ores). However,iron oxide-containing dusts or sludges resulting from iron and steelmaking processes may contain noxious Zn ingredients such as Zn oxide, toresult in a problem that iron sources of high Zn amount can not be usedas the starting material (hereinafter, used as the meaning of feedmaterial) for blast furnaces. In the blast furnace, if the startingmaterial contains a great amount of Zn, the operability of the blastfurnace is worsened, as well as it leads to a problem that Zn attacksrefractories of furnace walls.

As described above, use of dusts or sludges containing Zn-containingiron oxide has been restricted and, particularly, iron oxide-containingdusts or sludges containing Zn ingredients formed in a great amount fromblast furnaces have not been effectively recovered and reused as ironsources at present.

In view of the above, methods of producing reduced iron by removing Znfrom pellets which are molded from iron oxide-containing dusts orsludges resulting from iron and steel making processes have beenproposed in recent years. There can be mentioned, for example, a methodof using a rotary kiln furnace (refer to Japanese Patent ExaminedPublications Sho 51-13083 and 55-21810) and a method of using a rotaryhearth furnace (refer to Japanese Patent Unexamined Application Hei5-125454 filed in Japan corresponding to U.S. Pat. No. 5,186,741).

The method of using a rotary kiln has a merit in that Zn can bedecreased to as low as 0.01 mass % and metallization ratio can beimproved to about 80% (about 87% as a reducing ratio). However, sincepellets are tumbled and baked in a rotary kiln in this method, resultsin a problem that pellets are powderized during operation, which aremelted and deposited to form kiln rings, thereby making the operationimpossible. Furthermore, since the total amount of Zn in the startingmaterial fed to the blast furnace is controlled (for example, 0.2 kgZn/t-pig iron), and the amount of using reduced pellets containing Zn isrestricted, the amount of Zn has to be further decreased for use as thestarting material for the blast furnace also in this method.

Than, in a method of using a rotary hearth furnace described in JapanesePatent Unexamined Application Hei 5-125454, reduced pellets are producedby forming green pellets comprising a mixture of dusts from steel works,carbonaceous material such as coal or coke and an organic binder,feeding the green pellets on a layer of pellets baked in a rotary hearthfurnace, drying them at a temperature lower than 900° C. for 10 to 15min, thereby forming coked dried pellets and then reducing the pelletsat a temperature lower than 1150° C. for 20 to 30 min. While JapanesePatent Unexamined Application Hei 5-125454 describes for the reducedpellets that iron oxide is reduced to a metallic state and Zn ingredientis removed from the reduced pellets, it does not teach about the actualextent of reduction ratio and the amount of Zn. Furthermore, the methoddoes not define the contents of iron and Zn in the pellets and theamount of carbonaceous material to be added. As can be seen from thedescription that the reduced pellets, when discharged from a rotaryhearth furnace at about 1000° C., may possibly include a considerableamount of carbon (as high as 12 mass %) when they are discharged fromthe rotary hearth furnace), it is suggested that a great amount ofcarbon (as much as 12% by weight) may remain after the reduction in thismethod. Residue of a great amount of carbon brings about a problem thatnot only the content of iron in the reduced pellets is decreased tolower the utilizing efficiency as an iron source, but also the strengthof the reduced pellets per se is deteriorated. Particularly, when coalis added as the carbonaceous material, it tends to greatly deterioratethe strength of the reduced pellets per se. If the strength of thepellets is lower, it results in a problem that the reduced pellets arepulverized or crushed in the blast furnace, to lower the air ventilationin the blast furnace and worsen the blast furnace operation, so that thepellets can not be used as the starting material for the blast furnace.

SUMMARY OF THE INVENTION

An object of the present invention is to provide reduction pellets withless Zn content as noxious ingredients, having appropriate grain sizeand strength and with high reduction ratio as the starting material forthe blast furnace, by using iron oxide-containing dusts (includingsludges) formed from iron works, particularly, those blast furnace dusts(which contain carbonaceous material but can not be used as the startingmaterial for the blast furnace because of Zn ingredients contained andfinely particulate form thereof) and other dust containing Zningredients, as well as a method of producing them.

In the method of producing reduced pellets according to a preferredembodiment of the present invention, reduced iron pellets are producedby controlling a carbonaceous material such that the amount of carbon isfrom 7 to 60 mass % based on the total amount of iron and Zn in astarting material mixture containing one or more of dusts containingiron oxide and Zn oxide and a sufficient amount of a binder to bond thedusts and then adding water to prepare them into green pelletsincorporated with the carbonaceous material. Then, reduced iron pelletsare produced by drying the thus prepared green pellets into a reductionfurnace, heating the dry pellets by heat conduction, mainly, irradiationsuch that a temperature elevation rate is from 3 to 13° C./sec within atemperature range of the pellets from 150 to 900° C., reducing Zn oxideand evaporating Zn and reducing iron oxide.

In this case, the temperature elevation rate of the pellets is increasedand the amount of carbon charged in the pellets is optimized.Accordingly, the shape of the pellets upon reduction can be retainedand, as a result, the reduction ratio of the reduced pellets can beimproved and pulverization of the pellets can be prevented duringreduction.

Further, it is preferred to reduce iron oxides and Zn oxides at 1100 to1350° C.

In this case, since the Zn oxide is reduced preferentially to thereduction of the iron oxide at a temperature higher than 1100° C., theamount of Zn in the pellets can be decreased remarkably. Further, sincethe reduction ratio is improved and the sintering of metallic iron isproceeded, the strength of the reduced pellets can be increased.

Further, as the dusts, carbonaceous material-containing blast furnacedusts, converter dusts, sintering dusts, electric furnace dusts or amixture thereof can be used. Use of the dusts can decrease the amount ofindustrial wastes.

It is preferred to use a carbonaceous material comprising a cokeingredient.

In this case, since the coke has no substantial volatile ingredients,volatile ingredients are not evaporated in the above-mentionedtemperature range causing less reduction of pellets and sintering(150-900° C.). As a result, since the elevation of the gas pressurealong with the evaporation of the volatile ingredients is not caused andthe pellets are not pulverized, temperature elevation rate of thepellets can be increased. Particularly, when blast furnace dustscontaining the coke ingredients as the carbonaceous material are used,use of additional carbonaceous material such as coal or coke is no morerequired, so that energy and resource saving can be attained.

It is preferred to use a rotary hearth furnace hearth as the sinteringfurnace.

In this case, by the use of the rotary hearth furnace, reduced pelletscan be produced in a great amount and at a high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing compositions for wet blast furnace dusts androlling sludges in Example 4;

FIG. 2 is a table showing blending ratios of starting materials forsintering of wet blast furnace dusts and rolling sludges in Example 4;

FIG. 3 is a table showing reduction ratio and Zn-removing ratio ofreduced pellets produced in Example 4;

FIG. 4 is a conceptional view illustrating a structure of a rotaryhearth furnace used in the present invention;

FIG. 5 is a view showing effects of the amount of carbon on thereduction ratio of the pellets, Zn removing ratio and original shaperetainability of the pellets when they are heated in an inert atmosphereat 1300° C.;

FIG. 6 is a view showing effects of reduction temperature on thereduction ratio of pellets, Zn-removing ratio and compression strength;

FIG. 7 is a view showing effects of the temperature elevational rate ofpellets on the original shape retainability and the crushing strength ofthe reduced pellets in a case of heating the pellets at 1300° C. in aninert atmosphere;

FIG. 8 is a view showing a temperature elevation curve of the pellets;

FIG. 9 is a conceptional view illustrating a structure of an electricheating furnace used in the example of the present invention; and

FIG. 10 is a graph showing a relationship between the ratio (C/O) forthe amount of oxygen bonded with Fe and Zn in the pellets, and thereduction ratio of the reduced pellets, the Zn removing ratio and theoriginal shape retaining ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation is to be made to a preferred embodiment for the method ofproducing reduced pellets in a preferred embodiment according to thepresent invention. At first, a carbonaceous material is controlled suchthat the amount of carbon is from 7 to 60 mass % based on the totalamount of iron and Zn in a starting mixture comprising one or more ofdusts containing iron oxide and Zn oxide and a binder in an amountsufficient to bond the dusts, and water is added to prepare them intocarbonaceous material-incorporated green pellets. Then, the thusprepared green pellets are dried, the dry pellets are charged in areduction furnace, and the dry pellets are heated by heat transfermainly radiation such that the temperature elevation rate is from 3 to13° C./sec within a temperature range of the pellets from 150 to 900°C., thereby reducing Zn oxide and evaporating Zn, as well as reducingiron oxide to prepare reduced iron pellets.

It is necessary that the amount of carbon of the carbonaceous materialin the pellets is from 7 to 60 mass % based on the total amount of ironand Zn in the pellets. If it is less than 7 mass %, the amount of carbonrequired for the reduction of the iron oxide and Zn oxide in the pelletsis insufficient to lower the reduction ratio of the iron oxide and makereduction of Zn oxide insufficient thereby lowering the Zn removingratio and also lowering the pellet strength. On the other hand, if theamount of carbon exceeds 60 mass %, the pellet shape can no more beretained during reduction to lower the reduction efficiency.

Further, it is more preferred that the amount of carbon of thecarbonaceous material in the pellets is from 13 to 60 mass % based onthe total amount of iron and Zn in the pellets. This is because themetallizing ratio can be further improved at 13 mass % or higher. From13 to 45 mass % is further preferred. If the carbon amount is 45 mass %or less, the pellet reduction ratio can be increased and the pelletshape can be retained more firmly. 13 to 20 mass % is further preferred.If the carbon amount is less than 20 mass %, the blending amount of thecarbonaceous material can be decreased to save the amount of thecarbonaceous material, as well as the iron productivity is improved dueto the increase in the iron component.

The effect of the amount of carbon incorporated into the pellets will beexplained further. The pellet reduction ratio is determined depending onthe ratio between the amount of carbon and the amount of iron (amount ofiron oxide). Namely, iron components in the dusts or sludges as thestarting material for the pellets are usually present in the form ofiron oxide. In the reduction of the iron oxide, the iron oxide isreduced directly by the carbonaceous material present in the vicinitythereof, and the iron oxide is further reduced with CO gas formed fromthe reaction between CO₂ formed upon reduction of the oxides gas and thecarbonaceous material.

However, CO₂ gas formed upon reduction of the oxides and CO gas formedby reaction between the CO₂ gas and the carbonaceous material may oftenbeen discharged out of the pellets. Accordingly, it is preferred toincorporate the carbonaceous material into the pellets in an amountgreater than a stoichimetrical amount of carbon required for reducingthe iron oxides.

Furthermore, since the Zn ingredient in the pellets is often present inthe state of oxide like that iron oxide, it is necessary to furtherincorporate a carbonaceous material into the pellets in order to reduceZn oxide. Therefore, it is preferred to incorporate carbon into thepellets in an amount more than the theoretical amount of carbon requiredto reduce the iron oxide and Zn oxide. While the theoretical amount ofcarbon varies depending on the form of the reducing reaction of ironoxide and Zn oxide at the intended reduction temperature or the state ofthe oxides and it ranges from about 10 to 15 mass % based on the totalamount of iron and Zn in the pellets. Further, when a combustion flameof a burner is used for the irradiation heating of the pellets, if CO₂gas and steam (H₂O) evolved from the combustion are in contact with thepellet surface, reduced iron is oxidized again, so that it is necessaryalso for controlling the re-oxidation to incorporate the carbonaceousmaterial such that the amount of carbon is more than the theoreticalamount required for reducing the iron oxide (C/(Fe+Zn)). For thispurpose, it is desirable to control the amount of carbon in the pelletsto 13 mass % or more. However, when the more importance is attached tothe Zn removing ratio and physical strength rather than the reductionratio of products such as in a case of using pellets after reduction asthe starting material for the blast furnace, the amount of thecarbonaceous material to be blended can be decreased to 13% or less.This can provide a merit capable of decreasing the amount of thecarbonaceous material and improving the productivity.

On the other hand, if the addition amount of the carbonaceous materialincorporated in the pellets is increased, the shape retainability of thepellets is deteriorated to result in pulverization of the pellets duringreduction. Since the pulverization of pellets lowers the efficiency ofcontact of iron oxide and Zn oxide with the carbonaceous material or COgas, the reduction efficiency for the iron oxide and the reductionefficiency for Zn oxide are lowered. Furthermore, since the proceedingof the sintering for the pellets is inhibited making it difficult toretain the pellet shape, the original shape retainability of the pelletsis deteriorated remarkably. Accordingly, by defining the amount ofcarbon in the pellets, preferably, to 60 mass % and, more preferably, to45 mass % or less, the reduction ratio of the pellets can be furtherimproved and the shape of the pellets can be retained more firmly.

It is necessary that the temperature elevation ratio to 3 to 13° C./secwithin a temperature region of the pellets from 150 to 900° C. This isbecause the reduction of the pellets does not proceed so effectively,sintering for the iron ingredient particles less occurs and the strengthof the pellets is not increased within a temperature region from 150 to900° C. and, accordingly, it is necessary to rapidly pass thistemperature region from 150 to 950° C. rapidly. The strength of thepellets can be increased by bringing the pellets into a temperatureregion of 900° C. or higher where the reduction ratio of iron oxide ishigh and reduced iron particles in the pellets are sintered more rapidlywhile setting the temperature elevation rate to 3° C./sec or higher. Ifthe temperature elevation ratio exceeds 13° C./sec, the original shaperetainability of the pellets is deteriorated. The pellets are preferablyreduced at a temperature elevation rate of 5 to 10° C./sec. When thetemperature elevation rate is set to 5° C./sec or higher, the strengthof the reduced pellets can be further improved, and the shape can beretained more effectively. By increasing the temperature elevation rate,it is possible to rapidly reach a temperature region of 1100° C. orhigher where Zn is removed preferentially and, as a result, Zn can beremoved more efficiently with the carbonaceous material in the pellets.Particularly, when it is intended to remove Zn preferentially, thisrange for the temperature elevation rate is suitable. Further, thepellet productivity can also be improved by increasing the temperatureelevation rate for the pellets.

The pellets are preferably reduced within a temperature range from 1100to 1350° C. When the pellets are reduced at a temperature of 1100° C. orhigher, since Zn oxide is reduced preferentially to the reduction ofiron oxides, the amount of Zn in the pellets can be decreasedsignificantly. Further, if the temperature for reduction of the pelletsis at 1100° C. or higher, the reduction ratio is increased and thesintering for metal iron is proceeded to increase the strength of thereduced pellets. For example, the crush strength of the pellets reducedat 1100° C. is 70 kgf/P or more, and the value gives a sufficientstrength of pellets as the starting material for the blast furnace. Thisis because a value of 40 kgf/P or more is necessary as the startingmaterial for the blast furnace. The reduction temperature is preferablyset to 1200° C. or higher. This is because Zn oxide can be reduced morepreferentially and the strength of the reduced iron pellets can beincreased further. Furthermore, the productivity can be improved whilemaintaining the quality of the reduced iron pellets (Zn removing ratio,strength and iron reduction ratio). On the other hand, if the pelletsare reduced at a temperature higher than 1350° C., pellets tend to befused to each other, so that the pellet reduction temperature ispreferably set to 1350° C. or lower.

As the carbonaceous material incorporated into the pellets, cokeingredients are preferably used. Since coke scarcely has volatileingredients, it does not evaporate the volatile ingredients at atemperature region in which reduction and sintering of the pellets lessoccur (150-900° C.). Since this does not increase the gas pressureaccompanying the evaporation of the volatile ingredients and does notcause pulverization of the pellets, the temperature elevation rate ofthe pellets can be increased.

The sintering furnace has a furnace structure of interrupting the insideof the furnace from external air and preferably has a heating means forheating pellets fed in the furnace by heat conduction, mainly, radiationand a discharge means for discharging reaction gases containing the Zningredient reduced and evaporized from the pellets out of the furnace.As the heating means, radiation heat of burner combustion flames orradiation heat from radiant tubes can be used.

As the sintering furnace in the embodiment of the present invention, arotary hearth furnace is preferably used. By the use of the rotaryhearth furnace. Reduced pellets can be manufactured in a great amountand at a high efficiency. The structure of the rotary hearth furnace isto be explained with reference to FIG. 4.

The rotary hearth furnace has a disc-shaped hearth, on which pellets arefed and moved along with the movement of the hearth, thereby producingreduced pellets continuously. Since the pellets are moved together withthe hearth and the pellet themselves do not move, pulverization ofpellets during reduction as occurred in a rotary kiln can be prevented.Further, the rotary hearth furnace has a furnace structure in which theinside of the furnace and the external air are interrupted. As aninterrupting structure, heat resistant metal materials or refractoriesare used. Since the inside of the furnace is interrupted from theexternal air as described above, the atmosphere can be controlledeasily.

As shown in FIG. 4, a rotary hearth furnace 1 comprises a pelletcharging port 2, a reducing zone 4 and a reduced pellet discharge port5. The pellets are heated by heat conduction of radiation from a burner6 and a furnace wall. For heating the pellets, it is possible to use notonly heating by the burner but also radiation heat generated by puttinga CO gas resulting from the pellets to secondary combustion with air(shown at 7 in FIG. 4). As a result, it is possible to improve the heatconduction to the pellets and economize fuels for burner heating. Thecombustion gas is released from a gas discharge port 8 to the outside ofthe furnace. The combustion gas contains reaction gases containing Zningredients reduced and evaporated from the pellets, which form solid Znoxide due to lowering of the temperature and can recover and treat Znoxide by a dust collecting device.

Then, a method of producing reduced pellets in a rotary hearth furnaceshown in FIG. 4 will be explained as a preferred embodiment of thepresent invention. Pellets prepared by a pelletizer are dried in orderto improve the productivity of the rotary hearth furnace. The driedpellets are fed through the pellet charge port 2 to the rotary hearthfurnace. In this case, the pellets are laid by one layer or laminated bytwo layers on the hearth. In this case, pellets are not fused to eachother. It is considered that the pellets are merely laid by one layer orlaminated by two layers in the furnace and suffer from no substantialeffect of gravitational sintering caused by the lamination of thepellets to each other and that the pellets are not fused to each othersince there is less points of contact between each of the pellets.

In the reduction zone 4, the temperature of the pellets is rapidlyelevated by heat conduction of radiation through burner combustion andsecondary combustion, in which iron oxide is reduced and Zn oxide isreduced and evaporated with the carbonaceous material incorporated inthe pellets. In this case, since the amount of the carbonaceous materialfed in the pellets is more than the theoretical amount of carbonrequired for the reduction of iron oxide and Zn oxide (within a rangefrom about 10 to 15 mass %: although varying depending on the form ofthe compounds and the manner of reaction), when CO₂ and steam (H₂O)formed by burner combustion or secondary combustion reach the surface ofthe pellets, they are reduced to CO or H₂ with the carbonaceous materialincorporated in excess to the pellets, so that iron oxide and Zn oxidecan be reduced. Then, the reduced pellets are mechanically dischargedfrom the pellet discharge port 5.

In the reduction zone 4, the pellets are heated so as to be kept withina temperature range from 1100 to 1350° C., and heat is supplied for thetemperature elevation and endothermic reaction of the pellets in orderto reduce iron oxide and Zn oxide in the pellets. In this case, sincethe pellets are laid by one layer or laminated by two layers in thefurnace, heat conduction, mainly, radiation can be conductedefficiently.

EXAMPLE 1

In the embodiment of the present invention, were used dry pelletsincorporated with carbonaceous material which were prepared by mixing acarbonaceous material (coke ingredient), wet blast furnace dustscomprising iron oxide containing Zn ingredients and other dustscomprising iron oxide (for example rolling sludges) in a predeterminedblending ratio and putting them to molding and then drying steps. FIG. 1is a table showing compositions for wet blast furnace dusts and rollingsludges used in this example, FIG. 2 is a table showing blending ratiosfor sintering materials of the wet blast furnace dusts and rollingsludges and FIG. 3 is a table showing reduction ratio, Zn removing ratioand the like of reduced pellets produced by this example.

As shown in FIG. 1, the wet blast furnace dusts used comprise iron oxidecontaining 33 mass % of carbon as the coke ingredient and 3 mass % of Zningredient as impurities. The iron component and the Zn ingredient inthe blast furnace dusts are present substantially in the form of oxides.On the other hand, the rolling sludges contain 4 mass % of an oilcomponent and, like that blast furnace dusts, the iron component in therolling sludges is present substantially in the form of oxides. Suchblast furnace dusts and rolling sludges were mixed in a blending ratiosshown in FIG. 2, to which a binder and water were added and molded intogreen pellets with a diameter of about 16 to 20 mm. Then, the greenpellets were dried at 160° C. by a gas mixture comprising a combustiongas and air till the water content was lowered to less in 1.0 mass %.

Then, the dry pellets were charged in an electric heating furnacemaintained in an N₂ gas atmosphere at 1300° C. for 9 min. Thetemperature elevation curve of the pellets is shown in FIG. 8.

The sintering furnace used in the experiment of the present invention isan electric heating furnace 10 shown in FIG. 9 in which the inside ofthe furnace is interrupted from external air by a pipe 11 made of arefractory material. The carbonaceous material incorporated pellets wereset to a specimen holder 12 and charged into the electric heatingfurnace 10 previously maintained at a predetermined reductiontemperature in an N₂ atmosphere. Thermocouples (not illustrated) wereset to a central portion of the pellets in order to measure thetemperature elevation rate of the pellets.

At first, the effect of the amount of carbon incorporated into thepellets is explained with reference to FIG. 5. In FIG. 5, the abscissaindicates the amount of carbon incorporated in the pellets based on thetotal amount of iron and Zn in the pellets, while the ordinate indicatesthe reduction ratio of the pellets (in solid line) and the shaperetainability of the pellets (in dotted chain).

The reduction ratio of the pellets is a value obtained by dividing“amount of oxygen bonded with Fe removed by reduction” with “initialamount oxygen bonded with Fe”.

Further, the original shape retainability defines the appearance of thereduced pellets as:

Original shape retainability=Σ(shape index×frequency) %

Namely, the original shape retainability is expressed by defining theshape index as:

100: pellets retaining the shape, with no substantial cracks beingrecognized

75: pellets retaining the shape, with cracks being recognized

50: pellets including large cracks at the inside or cracked into twopieces

25: pellets cracked into several blocks

0: pellets being pulverized

and is expressed by multiplying the frequency exhibiting theabove-mentioned appearance with the shape factor.

As shown in FIG. 5, it can be seen that if the amount of carbon in thepellets based on the total amount of iron and Zn is within a range from13 to 60 mass %, the reduction ratio of the pellets is 75 mass % ormore, and the shape retainability of the reduced pellets is 50% or more.In this case, the Zn removing ratio is 98 mass % and it can be seen thatZn can be reduced satisfactorily.

Usually, it is required that the reduction ratio of the reduced pelletsis 75 mass % or more, and the original shape retainability of thereduced pellets has a limit for the shape retainability of 50% with aview point of the handlability upon charging them into a blast furnaceand a converter.

Furthermore, it is considered that if the amount of carbon in thepellets is 18 mass % or more, the reduction ratio of the reduced pelletsis 90 mass % or more, whereas if the amount of carbon in the pellets isless than 45 mass % or less, the original shape retainability of thereduced pellets is 75% or more. It has been confirmed that the strengthof the reduced pellets in this case is within a range from 45 to 80kgf/P and can be used as the starting material for the blast furnace.

EXAMPLE 2

Example 2 is an example showing the effect of the reduction temperatureof the pellets. The pellets were charged into the electric heatingfurnace kept at a temperature from 900 to 1350° C., maintained for 12min after the pellets reached a predetermined temperature and then thepellet reduction was conducted. FIG. 6 shows the results of measuringthe Zn amount, the reduction ratio and the crushing strength of thereduced pellets.

As shown in FIG. 6, the Zn removing ratio of the reduced pellets reached99 mass % or more at 1200° C. or higher, and the Zn amount was reducedto 0.01 mass % or less. This is because Zn oxide is reducedpreferentially to iron oxide in a high temperature region and it hasbeen proved that the amount of Zn is lower by one digit and Zn can beremoved more efficiently than in the prior art. On the other hand, itcan be seen that the reduction ratio and the crushing strength of thereduced pellets are increased along with the elevation of the reductiontemperature. They have a sufficient strength of the reduced pellets asthe starting material for the blast furnace at 1100° C. or higher. Inthis case, although fusion was partially recognized between each of thepellets at 1350° C., fusion was not recognized at other temperatures.Further, partially fused portions between each of the pellets at 1350°C. can also be pulverized mechanically.

EXAMPLE 3

Example 3 is an example showing the effect of the temperature elevationrate of the pellets.

For the pellets having the amount of carbon incorporated in the pelletsof 30.6 mass % and 53.8 mass %, respectively, based on the total amountof iron and Zn in the pellets, temperature elevation experiment wasconducted while varying the temperature elevation rate within atemperature range from 150 to 900° C. and the results are shown in FIG.7.

As shown in FIG. 7, it has been found for the amount of carbon of 30.6mass % in the pellets based on the total amount of iron and Zn, that theoriginal shape retainability of the pellets can be maintained to 50% atthe temperature elevation rate of the pellets of 13° C./sec or less, andthe original shape retainability of the pellets can be improved andshape retainability of the pellets is about 100% at the temperatureelevation rate of the pellets of 5° C./sec or less. In the same manner,it has been proved also for the amount of carbon of 53.8 mass % that theoriginal shape retainability of the pellets is improved along with thedecrease of the temperature elevation rate of the pellets, and theoriginal shape retainability of the pellets can be maintained to 50% orhigher by keeping the temperature elevation rate of the pellets to 10°C./sec or lower. On the other hand, along with the proceeding ofreduction, the crushing strength of 40 kgf/P is obtainable due to thesintering of the metallic iron at the temperature elevation rate of 3°C./sec or higher and the sintering becomes complete to obtain reducedpellets having a sufficient strength at a temperature elevation rate of5° C./sec or higher. Accordingly, it can be seen that a temperatureelevation rate of 3 to 13° C./sec, preferably, 5 to 10° C./sec isnecessary in order to obtain reduced pellets with high original shaperetainability and high strength.

EXAMPLE 4

Example 4 is an example showing the reduction ratio and the Zn removingratio of reduced pellets produced under the conditions of Example 1. Asshown in FIG. 3, it has been confirmed in any of examples of the presentinvention that the amount of Zn was reduced to 0.020 mass % or less andZn can be reduced satisfactorily.

In the example of the present invention, it can be seen that theoriginal shape retainability of the pellets is kept at 50% or more, thecrushing strength of the resultant reduced pellets is 45 kgf/P or moreand the reduction ratio is also 75 mass % or higher, which can be usedas the starting material for the blast furnace. Particularly, in a testmaterial 3 comprising blast furnace dusts and rolling sludges each at 50mass % blending ratio (amount of carbon in pellets based on iron+Zncomponent ratio), it can be seen that resultant pellets have a reductionratio of 99.1% and an original shape retainability of 98%, so that thereduction ratio and the original shape retainability of the pellets canbe removed remarkably.

In this example, the oil component was contained by 2 mass % in theinitially molded green pellets but, if the oil content in the greenpellet is about 2 mass %, breakage of pellets during reduction was notrecognized.

The method of the present invention can provide reduced pellets at highreduction degree (reduction ratio at 75 mass % or more) and withextremely small amount of Zn as the starting material not only for theblast furnace but also for the converter and the electric furnace.

The method of the present invention is not restricted only to thisembodiment, and the method can use not only the blast furnace dustscontaining the carbonaceous material and rolling sludges but also otheriron oxide-containing dusts or sludges resulting from iron and steelmaking processes as the starting material for the pellets. Furthermore,the grain size of the pellets is not restricted only to about 16-20 mmas shown in this example, but it may be about 6 to 16 mm which is ageneral particle size of sintered iron oxide pellets used for the blastfurnace. Furthermore, the method of the present invention can be usednot only to the reduced pellets but also to the manufacture of reducedbriquettes. Dry briquettes are used for the manufacture of the reducedbriquettes and dry briquettes are obtained by a method, for example, ofadding water to raw materials containing additives into briquettes andthen drying them like in the case of the green pellets, and a method ofbriquetting by adding additives to previously dried raw materials.

Furthermore, in this embodiment, the pellets were reduced in theelectric heating furnace shown in FIG. 9, and the results can be appliedto a rotary hearth furnace. When pellets are reduced in the rotaryhearth furnace, reduced pellets with extremely low Zn content, havingappropriate grain size and strength and with high reduction ratio can beproduced in a great amount and at a high efficiency.

In this embodiment, an inert gas typically represent by N₂ gas (alsoincluding Ar gas) was used as the atmosphere, but an atmosphere such asa combustion gas by burner heating as in a rotary hearth furnace mayalso be used. Since the combustion gas can be used as a carrier gas forthe reaction gas containing the Zn ingredients reduced and evaporatedfrom the pellets, there is no requirement for additionally supplying anN₂ gas or the like as the carrier gas, which can not only save N₂ gas orthe like but also avoid heat loss caused by N₂ gas supplied separately.

EXAMPLE 5

Further, since the sludges or dusts as the starting material for thepellets comprise, as the main ingredient, oxides of iron and Zn, andcomposite compounds of iron and Zn, the method of the present inventioncan also cope with a case in which a small amount of metallic iron,metallic Zn and composite compounds of iron and Zn are present. In sucha case, the amount of carbon in the pellets may be within a preferredrange of 13 to 60 mass % based on the total amount of iron and Zn. Thiscan be mentioned, in another expression, as that the ratio of the amountof the carbon blended in the pellets and the amount of oxygen bondedwith Fe and Zn in the pellets (C/O) is within a range from 65 to 160mass %. In view of FIG. 10 showing a relationship between the ratio ofthe amount of oxygen bonded with Fe and Zn in the pellets (C/O), and thereduction ratio, Zn removing ratio and the original shape retainability,it can be seen that the reduced pellets at high reduction ratio, withthe original shape retainability of 50% or more and with extremely lowZn content can be obtained within this range.

The entire disclosure of Japanese Patent Application No. 9-349473 filedon Dec. 18, 1997 including specification, claims, drawings and summaryare incorporated herein by reference in its entirety.

We claim:
 1. Reduced iron pellets produced by a method comprising: 1)preparing green pellets from a feed material mixture comprising one ormore dusts containing iron oxide and zinc oxide, a binder in an amountsufficient to bond the dusts, and water, wherein said green pelletsincorporate carbonaceous material from said one or more dusts and/oradded separately, and wherein said green pellets contain an amount ofcarbon in a concentration of from 7 to 60 mass % based on the totalamount of iron and zinc; 2) drying the thus prepared green pellets toprepare dry pellets; 3) feeding the dry pellets to a furnace; 4) heatingthe dry pellets at a temperature elevation rate of from 3 to 13° C./secwithin a temperature range from 150 to 900° C. of the pellets; and 5)further heating said pellets to temperatures sufficient to reduce saidzinc oxide to zinc and evaporate said zinc, and to reduce said ironoxide, wherein said reduced iron pellets have a strength of 45 kgf/P ormore, a reduction ratio of 75 mass % or more, and an original shaperetainability of 50% or more.
 2. Reduced iron pellets of claim 1,wherein the amount of the carbon is from 13 to 60 mass % based on thetotal amount of iron and Zn.
 3. Reduced iron pellets of claim 1, whereinthe amount of the carbon is from 13 to 45 mass % based on the totalamount of iron and Zn.
 4. Reduced iron pellets of claim 1, wherein thetemperature elevation rate is from 5 to 10° C./sec.
 5. Reduced ironpellets of claim 1, wherein reduction of the iron oxide and Zn oxide isconducted at a temperature from 1100 to 1350° C.
 6. Reduced iron pelletsof claim 1, wherein reduction of iron oxide and Zn oxide is conducted ata temperature from 1200 to 1350° C.
 7. Reduced iron pellets of claim 1,wherein the dusts are blast furnace dusts containing a carbonaceousmaterial, converter dusts, sintering dusts, electric furnace dusts ormixtures thereof.
 8. Reduced iron pellets of claim 1, wherein thecarbonaceous material comprises a coke ingredient.
 9. Reduced ironpellets of claim 1, wherein a rotary hearth furnace is used as thefurnace.
 10. Reduced iron pellets having a strength of 45 kgf/P or more,a reduction ratio of 75 mass % or more, and an original shaperetainability of 50% or more.